In 1985, it was reported that the synthetic nucleoside 3'-azido-3'-deoxythymidine (AZT) inhibits the replication of human immunodeficiency virus type 1 (referred to as HIV), the etiological cause of acquired immune deficiency syndrome (AIDS). Since then, it has been demonstrated that a number of other 2',3'-dideoxynucleosides, including ddC (2',3'-dideoxycytidine), ddI (2',3'-dideoxyinosine), 3'-fluoro-3'-deoxythymidine (FLT), and 2',3'-dideoxy-2',3'-didehydrothymidine (D4T) are active against HIV. It appears that, after cellular phosphorylation to the 5'-triphosphate by cellular kinases, the 2',3'-dideoxynucleosides exhibit activity by competitive inhibition of reverse transcriptase or by chain termination of a growing strand of viral DNA.
2',3'-Dideoxynucleosides have historically been prepared by either of two routes; deoxygenation and derivatization of an intact nucleoside, or condensation of a derivatized sugar moiety with a nitrogenous base (referred to below as the "convergent" method).
The condensation of a derivatized sugar moiety with a nitrogenous base is a more versatile approach to the preparation of 2',3'-dideoxynucleosides than derivatization of an intact nucleoside, in that it offers the opportunity to easily modify both the base and the sugar portions of the molecule. This facilitates structure activity evaluations of a range of potential anti-viral candidates. Further, it does not require an intact nucleoside as a starting material, which is often expensive and can be difficult to obtain in adequate quantities.
There are several difficulties encountered in the process of condensing a derivatized sugar moiety with a nitrogenous base. One problem is the direction of the stereochemistry of the glycosylation reaction. The .beta.-anomeric nucleoside is typically more biologically active than the corresponding .alpha.-anomeric nucleoside, and must be isolated from the mixture through an often difficult chromatographic separation.
A second problem encountered in the condensation reaction is that, since both purines and pyrimidines have more than one nitrogen in the ring, the condensation (glycosylation) reaction can occur at more than one site. It is typically desired that the N.sup.1 atom in pyrimidines and the N.sup.9 atom in purines become glycosylated to form the synthetic nucleoside. Under certain conditions, however, N.sup.3 glycosylated pyrimidines and N.sup.7 glycosylated purines are produced as side products. This lowers yield and necessitates additional purification efforts.
Many coupling reactions of purine or pyrimidine bases with ribose type carbohydrates employ the silyl variant of the Hilbert-Johnson procedure developed by Vorbruggen. Vorbruggen, H.; Kroilkiewicz, K.; Bennua, B. Chem. Ber. 1981, 114, 1234. This process involves the condensation of an oxygen and/or nitrogen trimethylsilylated pyrimidine or purine base with a ribose using trimethylsilyl trifluoromethanesulfonate (TMSOTf) as the Lewis acid.
2'-Deoxyribose has been coupled to purine and pyrimidine bases using the Vorbruggen method. However, little or no facial selectivity is observed in the glycosylation reaction due to the absence of the 2'-hydroxyl group in the ribose, that would otherwise direct the addition of the base to the .beta.-face of the sugar through a neighboring group effect. Chu, C. K.; Beach, W. J.; Ullas, G. V.; Kosugi, Y. Tetrahedron Lett. 1988, 29, 5349. The Vorbruggen glycosylation method has also been used with pyranosides (Vorbruggen, H.; Kroilkiewicz, K.; Bennua, B. Chem. Ber. 1981, 114, 1234), also without facial selectivity.
Two modest exceptions to this trend have been observed. Glycosylation of silylated thymine with a 3-fluoro-2,3-dideoxyribofuranoside results in a 5:1 stereoselectivity of the .beta.-anomer. Agyei-Aye, K.; Baker, D.C. Carb. Res. 1988, 53, 4780. Glycosylation of silylated thymine with the 3-O-ethyl-methyl sulfoxide ribofuranoside results in an 8:1 stereoselectivity of the .beta.-anomer. Okauchi, T.; Kubota, H.; Narasaka, K. Chem. Lett. 1989, 801.
Another problem in the preparation of 2'-deoxyribose nucleosides, identified by Vorbruggen (Vorbrugen, H.; Kroilkiewicz, K.; Bennua, B. Chem. Ber. 1981, 114, 1234), is that .alpha.- and .beta.-anomers equilibrate under the Vorbruggen reaction conditions, and therefore, enantiomeric excesses of the .beta.-anomer that are produced can be lost.
To overcome the lack of stereoselectivity in the glycosylation of 2'-deoxyribose derivatives with purine and pyrimidine bases, 2-substituted ribosides have been used in place of 2-deoxyribose, with the hope that the 2-substituent can control the selectivity of the glycosylation reaction. In early work in this area, Ryan, K. J., et al., J. Org. Chem. 1971, 36, 2646, reported that 2-benzylthioribose can be reacted with 6-chloropurine to provide a ratio of .beta. to .alpha. anomer of 2'.alpha.-thiobenzyl-2'-deoxy purine nucleoside of 2:1, in 38% yield.
Chu, et al., have reported that 2-.alpha.-phenylselenenyl ribose can be coupled with silylated thymine to provide a mixture of .beta.- and .alpha.anomers of the corresponding nucleoside in a 99:1 ratio. Chu, C. K.; Babu J. R.; Beach, J. W.; Ahn, S. K.; Huang, H.; Jeong, L. S.; Lee, S. J. J. Org. Chem. 1990, 55, 1418. In this reaction scheme, 5-O-protected-3,4-(dihydro) -3-(.alpha. and .beta.)-(aromatic or aliphaic)-selenenyl-(2(5H)-furanone is converted to 1-O-activated-2-(aromatic or aliphatic)-selenenyl-5-O-protected ribose which is reacted with a protected heterocyclic base in the presence of trimethylsilyl triflate or a Lewis acid to form a .beta.-anomeric nucleoside. While this reaction scheme provided very high .beta.-stereoselectivity, the process still required a two step procedure in which the 2-substituted ribose is prepared and purified and then reacted with the silylated base.
It has also been reported that a 2.alpha.-phenylthio moiety in a ribose will control the stereochemistry of the glycosylation reaction. Wilson, L. J.; Liotta, D. Tetrahedron Lett. 1990, 31, 1815. Kawakami, H.; Ebata, T.; Koseki, K.; Matsushita, H.; Naoi, Y.; Itoh, K. Chem. Lett. 1990, 1459. As shown in Scheme 1 below, for example, it was found that 2.alpha.-phenylthio-2',3'-dideoxyribofuranoside and silylated thymine can be condensed in the presence of stannic chloride to provide the desired .beta.-anomer of the nucleoside in good yield. The phenylsulfenyl group is then removed via oxidative elimination to give D4T (2',3'-dideoxydidehydrothymidine). In similar fashion, silylated N.sup.4 -acetyl-cytosine and uracil are coupled with high stereoselectivity. ##STR1##
The 2-substituted ribose used in this reaction is prepared from a protected-4-hydroxymethyl-2-phenylsulfenyl butyrolactone, which is initially formed as a mixture of isomers (2-1:1, trans:cis) through formation of the enolate. The production of a mixture of isomers lowered yield and necessitated purification steps. It was later discovered that the desired trans protected-4-hydroxymethyl-2-phenylsulfenyl butyrolactone can be formed in much larger selectivity (9-22:1) and better yields by using the silyl ketene acetal and N-phenylthio-amides. This improved the overall stereoselectivity of the process, however, the process still required a two step procedure in which the 2-substituted ribose is prepared and purified and then reacted with the silylated base. Further, this procedure cannot be used for the preparation of purine nucleosides because a mixture of N.sup.7 and N.sup.9 glycosylated purine isomers are obtained.
Kim, et al., have recently reported that 2',3'-dideoxy and 2',3'-dideoxy-2',3'-didehydronucleosides can be prepared with high .beta.-stereoselectivity by condensing a furanoid glycal (5-(S)-6-(protected-oxy)-4,5-dihydrofuran) with a purine or pyrimidine base in the presence of Niodosuccinimide. Kim, et al., Tetrahedron Letters, Vol 33(39), 5733-5736 (1992).
Given the pharmaceutical importance of 2',3'-dideoxynucleosides, 2'-deoxynucleosides, and 2',3'-dideoxy-2',3'-didehydronucleosides, it would be of significant benefit to have a facile process for their production in which the base is glycosylated with high .beta.-stereoselectivity without the formation of undesired N.sup.7 glycosylated purines or N.sup.3 glycosylated pyrimidines. It would also be of benefit to have a process for the preparation of 2',3'-dideoxynucleosides and 2',3'-dideoxy-2',3'-didehydronucleosides that can be easily modified to increase the stereoselectivity of formation of the .beta.-anomer as necessary.
It is therefore an object of the present invention to provide a process for the preparation of 2',3'dideoxynucleosides 2'-deoxynucleosides, and 2',3'-dideoxy-2',3'-didehydronucleosides in which a purine or pyrimidine base is glycosylated to form a .beta.-anomeric nucleoside with high stereoselectivity.
It is another object of the present invention to provide a process for the preparation of 2',3'-dideoxynucleosides, 2'-deoxynucleosides, and 2',3'-dideoxy-2',3'-didehydronucleosides in which a stereoselecting moiety can be positioned in the ribose to direct glycosylation without the need for a separate isolation and purification step.
It is a further object of the present invention to provide a process for the preparation of 2'-deoxy purine nucleosides that results in a high yield of the N.sup.9 -glycosylated isomer.
It is another object of the present invention to provide a process for the preparation of 2',3'-dideoxynucleosides, 2'-deoxynucleosides and 2',3'-dideoxy-2',3'-didehydronucleosides that can be easily modified to increase the stereoselectivity of formation of the .beta.-anomer as necessary.